Network Monitoring

This is called eavesdropping or (in some circles) packet snarfing. Once the host has copies of all the frames it desires, it can then analyse them to discover the data they contain.

Most data transfers across the Internet are not encoded (or encrypted) in any way Ð the data is simply sent as plain text. Thus it is simple to observe messages transmitted by others. This is the origin of the (oft repeated, and generally true) assertion that "The Internet is insecure".

An area where this insecurity can present a serious problem is password authentication. At Bendigo, students can use the TELNET protocol to connect to the various Unix systems. The password which is typed by the student is transmitted across the LAN as plain text, and can be observed by any other student using freely available PC software. You need to always be aware of this!

Encryption

Encryption security based on:

• encryption algorithm - must be complex enough to make it impractical to decrypt
• secrecy of key(s) - most important
  • open encryption Ð publicly described
  • closed encryption Ð private vendors, classified products of security agencies

• key applied to plaintext to produce ciphertext. The algorithm by which the key is applied is critical element.
• key applied to ciphertext to produce original text at receiver - most systems use same algorithm as encryption, applied "in reverse".

• secrecy of key is principle problem
• Also: key distribution schemes
• Trusted brokers, etc

Single Key Systems - Vernam Cipher

• The very best encryption technique
• message represented as a string of bits
• key (random sequence of bits) applied to text (key is as long as text)
• re apply same key to ciphertext to produce original text
• practically impossible to recover original text without key
• Hard to implement in practice.
• Some one-time systems are used: see the skey utility, soon to be installed on our Unix systems.

Data Encryption Standard (DES)

• data encrypted into 64 bits blocks using 56 bit key
• ÒtheoreticallyÓ impossible to crack...

• input block of text (64 bit)
• 64 bit key (56 bit key + 8 bit parity)
• switch (encrypt / decrypt)
• output block of text (64 bit)

Clipper

• "Big Brother is watching..."
• "It is a closed system that gives up all pretense of allowing the private sector to keep information private from the government" Stallings & Van Slyke 1994:599
• Clipper chip is hardware encryption device based on the skipjack encryption algorithm, regarded as very strong. It has been mandated for use in the US, eventually replacing DES.
• Chip users (manufactuers?) register key with government: key escrow. The key is split into two halves, which are stored independently with different government agencies.
• Court-approved "wiretap" (ie, decryption) operations require both halves of the key to access any data transmissions. In addition, a released key can be coded so that it only works for a limited time period.

• Privacy concerns
• some doubt about whether snooping decryption could be avoided by clever users.

Public Key Systems

• "public" key to encrypt + private key to decrypt
• each node/station generates a key pair
• each node / station publishes their public key and secures their private key
• Node A sends to node B, encrypts message using node B's public key
• Node B decrypts message using its private key
• algorithms are extremely complex
• problems of authentication of public keys, compromised keys, bogus & out of date keys
• See question 4 of assignment 3 if you wish to pursue this in more detail...

Encryption Management

• link encryption Ð one encryption device per link, problem in packet switched network

• manual delivery
• transmit new key using old key
• end to end encryption

• monitoring traffic is a problem, hacker can spot volume & make assumptions
• traffic padding produces meaningless ciphertext in the absence of message text - a continuous stream of data is generated for the entire duration of each transmission

Encryption Applications

• Data to be sent, together with a secret key, are used to generate a message authentication code
• Data plus code are transmitted to recipient
• Recipient performs same operation, using secret key. If the code produced matches, message is authenticated.
• Can use non-reversible encypherment (eg, Unix /etc/passwd encryption)

• Message authentication protects message from interference by a third part, but does not guarantee shource fo the message.
• Use public key system to guarantee that message is, in fact, from the person who claims to have sent it.